Compact storage of seat and coolness by phase change materials while preventing stratification

ABSTRACT

While many materials and additives which will melt and freeze at various temperature levels for storing and releasing large amounts of heat thereby per unit volume have been disclosed, the packaging of these materials with suitable non-corrodible long-lasting heat exchange structures has been cumbersome and expensive. The present invention provides an inexpensive, high performance, non-corrodible thermal storage method and system adapted for use with heat storage materials of various compositions and adapted for use over a wide range of temperatures, including a heat exchanger which provides for phase change to occur approximately simultaneously throughout the volume of the entire storage mass and provides for the sites at which the phase change is occurring to be approximately uniformly distributed throughout the volume of the heat storage material. Problems of thermal expansion, stratification and sub-cooling are eliminated. Thermal storage methods and systems embodying the present system may advantageously be used for off-peak storage of electric refrigeration, cooling and heating as well as solar heating and other applications.

RELATED APPLICATIONS

The present application is a continuation of prior copendingapplication, Ser. No. 923,984, filed July 12, 1978, and which was issuedas U.S. Pat. No. 4,294,078, on Oct. 13, 1981; and said prior copendingapplication was a continuation-in-part of an earlier copendingapplication, Ser. No. 790,919, filed Apr. 26, 1977, and now abandoned.

BACKGROUND OF THE INVENTION

The bulk storage of heat or coolness at certain temperature levels hasmany applications, such as in solar heating of buildings at 85° to 120°F., in solar Rankine engines or absorption refrigeration machines at200° to 250° F., in off-peak hour operation of air conditioners at 30°to 60° F., in off-peak hour operation of refrigeration plants at -20 to+20° F., etc. The heat storage materials used, except in the case ofwater at 32° F., must be carefully mixed in certain proportions withspecial equipment and techniques, and must be kept away from materialsthat will corrode. Such heat storage materials are bulky and heavy totransport, and must be used in contact with large area heat exchangedevices because of the poor thermal conductivity of these heat storagematerials.

In order to minimize volume, weight, and cost, heat of fusion materialswith change of phase between solid and liquid have been proposed, testedand tried experimentally because 7,000 to 12,000 BTU's per cubic footcan be stored within the above narrow temperature ranges, whereas ifonly a liquid phase is used, such as water, capacity is limited to 2,000to 3,000 BTU's per cu. ft. or so. These heat of fusion materials,particularly the sodium and calcium salt hydrates, must have provisionsto prevent stratification.

Most prior designs have used air as the heat transfer medium. Such priorart designs have been very bulky due to the required volume of the airducts and also have required multiple encapsulation because of therequisite multiple air passages of comparatively large cross-sectionalarea. In certain instances, the prior art has attempted to utilizeliquid as the heat transfer medium; however, such prior art arrangementshave been largely limited to the freezing of water in metallic tanks,plates or tubes. However, such metallic structures suffer from corrosionand cause galvanic action which can cause rapid deterioration of variouscomponent parts. Moreover, they are expensive and inflexible, subject todamage due to expansion of the phase change material, and are heavy anddifficult to transport. Other suggestions have involved multipleencapsulation with its consequent high cost.

Such prior art multiple encapsulation techniques have generally usedsmall containers whose walls are not insulated, because the heattransfer must occur through the container walls. The uninsulated smallcontainers are inefficient since undesired heat loss or heat inflow canreadily occur through the uninsulated walls during periods of storage ofheat or coolness. This problem of inefficiency of the uninsulatedcontainer is augmented by multiple small containers, because theyinherently have a relatively large surface-to-volume ratio. Moreover,during the transfer of heat energy into the encapsulated containers, thePCM begins melting near the uninsulated wall of each container. Theinterior region of the PCM is the last to melt. Consequently, duringmost of a heat storing sycle, the larger proportion of the stored energyis relatively close to the container wall. In other words, a non-uniformdistribution of the stored energy often exists, with more being stored,on average, near the uninsulated container wall where it is relativelyeasily lost to ambient. Convection of the melted PCM adjacent to theuninsulated container wall carries away heat energy during storageperiods.

SUMMARY OF THE INVENTION

There are eight primary problems that have heretofore prevented the useof heat of fusion materials, or so-called phase change materials, frombeing used in the storing of thermal energy in a practical manner. Theyare cost of equipment, poor thermal conductivity of phase changematerials (PCM's), corrosion, volume change during fusion, evaporationof water from salt hydrates, subcooling and stratification of suchmaterials, and cost of shipping. The way that the present inventionsolves these eight problems is enumerated below.

1. Cost of equipment: The first advantage of the present invention isthat it enables the use of plastic transfer tubing whose relatively lowthermal conductivity can be compensated for by greatly increasing theheat transfer surfaces in accordance with the method and system of thisinvention thereby providing a large saving in cost. One or only a fewplastic tanks are used instead of multiple encapsulation, thus alsolowering the cost.

2. Poor Thermal Conductivity: The limitation in heat transfer rates,moreover, is not in the liquid conduit material but in the body of thephase change material. Thus, the large amount of plastic heat transferarea is matched with characteristics of the PCM by a multiplicity ofsmall plastic liquid transporting tubes distributed uniformly throughoutthe entire mass of the PCM. The heat flow path at any point is thus madevery short.

3. Corrosion: Corrosion is of particular importance because inorganicsalt hydrates provide the necessary medium for a battery if twodissimilar metals are present in any form within the salt. Severecorrosion of the metals can result quickly. Plastics alone, includingtubing, headers and fittings, or with but one non-corrodible metal suchas stainless steel anywhere within the salt can be satisfactory.

4. Volume Change: The problem of volume change during fusion is greatlylessened by having a flexible plastic material for both the outercontainer for the PCM and for the heat transfer surfaces all throughoutthe PCM. They will take up any thermal expansion forces both on a largescale and also locally in connection with a particular tube.

However, an element of this invention is that the plastic tubes withinthe PCM are arranged so that the average temperature between the liquidin any point in any tube throughout the PCM and that in the adjoiningtube is approximately the same. This is accomplished by means ofmultiple parallel circuits with U-bends at the end of each circuit andevery alternate tube connected to a supply header and the adjoiningtubes to a return header. See patent to C. D. MacCracken and HelmutSchmidt, U.S. Pat. No. 3,751,935 dated Aug. 14, 1973, for a method ofcreating an ice slab of uniform temperature for ice skating rinks whichhas since become the leading way to build an ice rink in the UnitedStates, referred to commercially as the Icemat rink.

When water is frozen to ice, which is one of the many PCM's utilized inthis invention, the heat transfer liquid enters the supply header andsmall tubes typically at about 24° F. and leaves the small tubes andreturn header at about 32° F. With a small plastic tube at 24° F.adjoining one at 32° F., the average temperature is 28° F. and ice willform at a rate caused by that average temperature. Half way to theU-bends in each parallel circuit, the temperature in the supply tubewill be 26° F. and the adjoining return tube, 30° F., giving the sameaverage temperature of 28° F. At the U-bends, where the supply andreturn small plastic tubes are joined, the temperatures will be 28° F.in both.

Therefore, ice advantageously builds uniformly on all tubes entirelythroughout the whole tank of water. The water level rises in the tankbecause of the increased specific volume of the ice formed but there isno sideward expansion forces as the ice joins from one spiral layer tothe other because the extra water volume has been squeezed upwardspreviously. The rise in water level provides a measure of the extent ofthe fusion process. The extra water on top is the last to freeze.

Similarly, in other PCM's, the volume change is accommodated withinthermal forces. Generally, it is a fact that PCM's with a melting pointabove 32° F. shrink when they solidify and for 32° F. and below theyexpand. For the PCM's that shrink when they solidify, the tank is filledwith liquid phase PCM above the level of the tubes by the amount of thevolume change shrinkage. When shrinkage during fusion occurs the saltadvantageously stays in contact with the heat transfer surfaces becauseof the bulk weight which is not the case in encapsulated trays or tubeswhere contact is lost between the upper heat transfer wall and the saltbecause of shrinkage causing poor heat transfer efficiency.

5. Evaporation: Evaporation of water from salt hydrates changing theircomposition and thermal performance takes place even through sealedplastic walls because of the property of plastics called "water vaportransmission". This means that salt hydrates sealed into multiple smallplastic containers will eventaully change in performance with no way torepair this except by replacement. In the present invention, only one orat most a few, relatively large containers are used to hold the salthydrates with removable covers, so that water may be poured in to refillthe lost water evaporated up to a mark showing the proper level.

6. Subcooling: A major problem of salt hydrates is subcooling, droppingbelow the freezing point without crystallizing or freezing taking place.This occurs because all the crystals are melted when the salt hydrate isheated above the melting (freezing) point and these crystals are notpresent to seed or nucleate upon recooling. Additives have beendiscovered for many of the salt hydrates to promote nucleation (see U.S.Pat. Nos. 2,677,644 and 2,936,741 to M. Telkes). Another very simplemethod is practical in the case of the present invention where the salthydrates with melting points above room temperature are held in a fewlarge insulated tanks. A very small projection from the tank outside ofthe insulation keeps the salt hydrates in this projection, or finger,from melting when heat is applied. Thus, the frozen crystals are presentat all times and will nucleate crystallization when the salt hydrate iscooled below its freezing point. For example, the velocity ofcrystallization of sodium thiosulfate pentahydrate is about one inch perminute, so a 4 ft. diameter tank nucleates throughout in an hour or lesswhen cooling is provided by the heat transfer liquid.

7. Stratification: Another problem of salt hydrates which this inventionovercomes is the stratification of solid crystals which, being heavier,in the case of most salt hydrates, sink to the bottom. They oftenmucleate into different hydrate molecule combinations as they fallthrough warmer areas. Because of incongruent phase change in thesedifferent hydrate molecules, the overall composition is changed andconsequently the performance. Also, a permanent precipitate forms at thebottom. On solution to this has been to limit the vertical dimension toan inch or so. In the present invention, a straw-like mat of rubberizedhair or other inert low density matting is used as a spacer between thetubes. This effectively fills all the space in the tank with such smallopenings like a filter that there is no room for crystals to fallthrough. In addition, since the dual tubing average the temperatureuniformly throughout, the crystal growth will be also uniform throughoutand there will be no temperature differences to cause large crystalbuild-ups in one area over another.

In a presently preferred embodiment of the invention, a small,inexpensive, low-power circulating pump and conduit apparatus serve tokeep the melted phase change material in circulation within therelatively large heat storage tank. Thus, stratification does not occur.

8. Shipping Cost: An average solar heated house is required to store onemillion BTU's, and at 100 BTU's per pound, five tons of PCM is neededand must be shipped. The present invention provides for the PCM to beshipped in heated tank trucks and pumped through a hose in liquid forminto the lightweight tanks and heat exchange tubing units which havebeen previously installed and tested. It is well known that use of tanktrucks is a much more economical method of transporting and deliveringlarge volumes of liquid to many delivery points than by sealedcontainers.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be better understood by reference to the drawings.

FIG. 1 is an elevational view of the thermal storage device showing acylindrical, open-top tank with spiral tubing and spacer mats in crosssection;

FIG. 2 is an elevational view of the flexible tubing grid and spacermatting being rolled up into a heat exchanger assembly;

FIG. 3 is a sectional plan view of the spiral tubing grid and spacermatting rolled up and installed in the thermally cylindrical tank;

FIG. 4 is an elevational sectional view similar to FIG. 1 of the thermalstorage apparatus showing a rectangular tank with tubing mats tensionedaround spacer bars and running up and down vertically;

FIG. 5 shows a schematic circuit diagram illustrating a thermal storageapparatus connected to piping to a pump, heat input and output devices;

FIG. 6 shows an enlarged partial sectional view of a portion of theapparatus of FIG. 1 showing tubing, spacer material and phase changematerial in a plurality frozen condition;

FIG. 7 is an elevational view of the thermal storage device similar tothat shown in FIG. 1 but with a circulating pump for preventingstratification;

FIG. 8 is an elevational view of the flexible tubing grid being rolledup using tubing spacers as an alternate to the use of matting as shownin FIG. 2; and

FIG. 9 is a sectional plan view of the spiral tubing grid made inaccordance with the procedures shown in FIG. 8 and installed in athermally insulated tank.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS

Referring now to the drawings in greater detail, FIG. 1 illustrates aphase change thermal storage unit 2 which stores and releases a largeamount of heat and coolness per unit volume by virtue of the heatcapacity of the phase change materials. There are many phase changematerials and additives which, when phase change takes place, release alarge amount of heat during freezing and absorb a large amount of heatduring melting. Examples of such PCM's are indicated further below.

The thermal storage device 2 consists of a semiflexible walled open-toptank or container 4 which preferably is made of thermoplastic materialto provide flexibility and resistance to corrosion, an important factor.Most phase change materials (PCM's ) are corrosive to metals.

Prior to filling the tank with PCM 24, a preformed roll of a flexibletubing mat 8 and rubberized hair 22 are placed in the tank. This roll ofmat 8 and spacing means, for example a mat 22 of rubberized hair, fillthe space in the tank, so that no region within the entire tank is morethan a short distance away from the mat tubing which carries the heattransfer liquid 26 for heating (melting) and/or cooling (freezing) ofthe PCM. The heat transfer liquid 26 may be water, or if used below 32°F. (0° C.), an antifreeze solution must be utilized, such as ethyleneglycol mixed with water. The flexible tubing mat 8 is prefabricated inthe factory using extruded twin tubings of small diameter, for examplehaving an inside diameter of approximately one-quarter of an inch,usually made of synthetic plastic material, for example such as ethylenevinyl acetate (EVA), which are kept closely spaced and parallel to oneanother by means of a spacer strip assembly which consists of a rigidplastic strip 12 and a flexible plastic strip 14 attached together insuch a way that they form tight pockets for twin tubings 10.

A more popular method of pre-forming this grid of mat 8 prior to rollingit up is by heat sealing. For uses involving temperatures too high forcommon plastics, synthetic rubber or elastomeric compounds, may be usedsuch as EPDM (ethylene propylene diene terpolymer). A number of twintubings or dual tubes 10 (number of tubings is dependent on the width ofthe mat grid, a popular width is 4 ft. nominal which includes 32 twintubings spaced approximately one and a half inch from the centerline ofone pair of tubes to the centerline of the next pair) are placed at agiven spacing and arranged parallel to one another. A rigid vinyl strip12 is placed under the twin tubings 10 and a flexible vinyl strip 14 isplaced over the twin tubings 10 in such a way that it is located rightover and aligned with the rigid strip 12. These two strips 12 and 14 arethen heat sealed together between the dual tubes, so that they formsupporting loops or pockets 15 around the respective twin tubings. Asseen in FIG. 2, the supporting strips 12 and 14 are positioned atapproximately evenly spaced locations along the length of the grid-likeflexible tubing mat 8 and are heat sealed together as described abovebefore the mat is rolled up in order to provide support for the twintubings 10, as seen enlarged in FIG. 1. These mats 8 may be fabricatedto any desired length. At one end of the mat 8, two headers areinstalled, --one is supply header 16, and the other is return header 18.On the other end of mat 8, the `U` bends 20 are installed forinterconnecting the tubing ends.

Referring to FIG. 2, the flexible tubing mat 8 and spacing means 22shown as rubberized hair matting are rolled together by laying out on along table, a longer length of the rubberized horse hair matting 22 ontop of the flat extended flexible tubing grid 8. Then a roll is formedstarting from the `U` bend end of the mat keeping the starting circle assmall as possible (about 2"-4" diameter). When one completes the rollingprocess, a roll is formed which has alternate layers of the flexiblegrid of tubing 8, and a spacer medium in the form of a flexible fibrouslow density material having relatively large spaces between fibers, forexample, a rubberized hair 22 is used like in pole vault and highjumping landing pits. This spacer means 22 serves to space apart thesuccessive convolutions of the rolled-up flexible grid-like tubing mat8. The supply header 16 and return header 18 are on the outside of theresulting roll, but as can be seen in FIG. 3, an extension of the layerof spacer matting serves to separate the tubing from the tank wall. Thisroll is then installed in the tank 4. Two ports are provided in the tankwall for inlet connection assembly 27 and an outlet connection assembly28 which are connected to the supply header 16 and return header 18 bymeans of known piping and plumbing methods. For large tanks more thanone tubing mat 8 may be placed end-to-end for the rolling procedureshown in FIG. 2, and rolled up in one large roll. It is preferred to usea plurality of such tubing mats 8 in one large roll rather than using asingle mat of greater length, so as to limit the pressure drop of theheat exchange liquid 26 circulating in the tubing.

The device 2 is filled with a suitable PCM 24 in accordance with thetemperature requirements of the application. The tank 4 is kept coveredusing a clamped plywood or plastic cover 6 and gasket 9 as a preventionagainst dust accumulation, evaporation and contamination of the PCM 24,and also to keep the light weight buoyant heat exchanger from raising upout of the often heavier density PCM when in the liquid state. The coverclamps are shown at 37, and a removable plug 25 is shown in the cover.Insulation 32 is provided all around the tank 4. The tank 4 is alsoprovided with a nucleating element in the form of a tubular conduitwhich protrudes outside the insulation 32 and is exposed to ambienttemperature. The purpose of nucleating device 30 is to retain somefrozen crystals of the PCM 24 while all the PCM 24 inside the tank 4 isin molten state. These trapped crystals in the nucleating device 30 arevery helpful in initiating the crystallization of the PCM for avoidingsubcooling. It is a tendency of the liquefied PCM, when there are nocrystals present within the material, that it becomes subcooled, whichis undesirable because it delays the phase change and reduces itseffectiveness.

FIG. 3 shows a sectional plan view of FIG. 1, to show how the spacermatting is located with respect to the tubing grid 8. Depending upon theheat transfer cycle time for charging and discharging the PCM storagematerial, a thinner or thicker matting may be selected with consequentchange in length of tubing grid and matting. Details of this arediscussed later. From the description of FIG. 1, the details of theelements may be understood.

FIG. 4 illustrates another thermal storage device 2. The device includesa rectangular tank 5 which is preferably made of a thermoplasticmaterial which thereby provides its quality of being semi-flexible andresistant to corrosion to various chemicals used as PCM's. However, thetank may be metallic, particularly if the PCM is water, and a plasticliner or coating is used over the metal. The rectangular design of thetank 5 facilitates the use of flexible tubing mat 8 without the use ofspacer material throughout the length of the mat 8. The mat 8 isinstalled differently by festooning the mat up and down around thespacer rods 40 which are installed in two rows, one near the top of thetank 5 and the other near the bottom. In each row spacer and supportrods 40 are equally spaced, for example at approximately 11/2" to 6"center-to-center distance and are arranged parallel to one another.Supply header 16 and return header 18 are located on the flange in theupper portion of the tank 5 where they are secured in place by headerholding clips 38 which are shown attached to the flange wall of the tank5. The other end of the mat 8 which has `U` bends 20 is located on theflange of the upper portion of the tank on the opposite side from theheaders. The `U` bends 20 are secured in place on the flange by using ananchor strip 36 which has the same number of hooked fingers as the `U`bends. After assembly, the tank 5 is filled with the PCM 24. The tank 5has a cover 7 to prevent evaporation and contamination by fallingforeign matter but need not be the clamped cover of FIG. 1, because thetubing assembly is anchored by spacer rods 40. The tank is located on aninsulated surface 34 and is well insulated all around with insulation32. A nucleating device 30 extends through the insulation 32 outside thetank. As described under FIG. 1, this nucleating device 30 retains somecrystals while all the other PCM 24 is in molten state inside the tank5. These trapped crystals in the nucleating device 30 help start thefreezing cycle without undergoing subcooling.

FIG. 5 shows a schematic diagram of a thermal storage system with a tank50 containing a PCM for storing thermal energy by the latent heat offusion, a pump 52 for pumping heat transfer liquid 26 through multiplesmall tubes 8 and 10, as shown in FIG. 1, and a variety of heat inputdevices on the left side and heat output devices on the right sideconnected with piping 53 and valves 55; some of the valves are not shownbecause it will be understood by those skilled in the art how to selectvarious piping circuits to and from the respective heat input and heatoutput devices by valving. FIG. 5 depicts an illustrative system only inwhich many different heat input devices and heat putput devices areshown for purposes of explanation. In any given installation, only theparticular desired input and output devices are included, as will beunderstood. The valves 55 are shown as being thermally actuated, and therespective temperatures at which they become actuated, i.e. opened andclosed, are selected in accordance with the operating temperature rangeof the respective heat input devices and heat output devices utilized inthat given installation. Also, as will be explained in detailhereinafter, the PCM to be used in the heat storage tank 50 is chosen,so as to have a melting temperature which is suitable for the particularheat output device and input device being utilized. For example, aradiant floor heater 40 has a relatively great effective area and,therefore, operates to advantage when the liquid 26 being circulated isheated to a moderate temperature by the PCM in tank 50. On the otherhand, a baseboard heater 78 has a much lesser effective area and,therefore, operates to advantage when the liquid 26 being circulated isheated to somewhat higher temperature by employing a PCM in the tank 50which melts at a correspondingly higher temperature. This selection of aPCM having a melting temperature suitable for a particular installationhaving particular heat input and output devices will be explainedfurther by specific examples.

For example, starting at the top left and proceeding down are shownexamples of heat input equipment: solar collector 54, air coil 56, heatpump 58, electric resistance liquid heater 60, fossil fuel boiler 62,ice skating rink grid 64, and cold storage or freezer room 66.

Starting at top right and proceeding down are shown examples of heatoutput equipment including: agricultural or industrial process heater70, water heater 72, heating coil in air duct 74, heat pump 76, radiantbaseboard heater 78, radiant floor heater 80, Rankine engine 82,absorption air conditioner 84, and air coil or cooling tower 86.

The various heat input devices shown at 54-66 may all be used to melt aPCM in tank 50 selected for the appropriate temperature level of thatheat input device. For example, solar collector 54 may heat fluid 26 to130° F. as it is being pumped through collector 54 on its way to tank50. Pipe insulation 57 prevents substantial change in temperature ofliquid 26. The reader will understand that such pipe insulation 57should be distributed throughout the system but is omitted to simplifythe drawing. In tank 50 solid PCM sodium thiosulfate pentahydrate, whichmelts at 118° F., will begin to melt by taking heat from fluid 26,dropping its temperature from 130° F. or slightly less to perhaps 125°F. as it leaves tank 50 and is pumped back to collector 54. Afterseveral hours, the length of time depending on the total area andspacing of small tubes 8 and 10, the PCM will be totally melted exceptfor what is in nucleating device tube 30 (FIG. 1).

The heat stored in the above example in tank 50 by a PCM which melts at118° F. may be pumped via liquid 26 to various heat outlet devices onthe right side when desired by suitable valve operation. For example,the heating coil in air duct 74 may be selected and heating providedthereby to a structure, not shown, in the usual manner by warming of airwhich is then circulated through air ducts in the building structurewhich is to be heated. Or water heater 72 may be heated by fluid 26.Similarly, the agricultural or process heater 70 may be heated bycirculating the warm heat transfer liquid 26, for example for anagricultural use, such as for performing grain drying, or the floorradiant heater 80 may be heated by circulating the warm liquid 26.

If a higher temperature PCM were selected, for example such as trisodiumphosphate dodecahydrate which melts at 150° F., or magnesium chloridehexahydrate which melts at 243° F., solar collector 54 could be utilizedadvantageously to supply heat to heat output devices such as thebaseboard radiant heater 78, Rankine engine 82, and absorption airconditioner.

Heat pump 76 could be best utilized for example with a PCM which meltsat 32° F., such as water or a PCM which melts at 55° F., such as sodiumsulphate decahydrate mixed with chloride salts as discussed in MariaTelkes U.S. Pat. No. 3,986,969. In other words, such a PCM having arelatively low melting temperature in the range of approximately 32° F.to 55° F. is advantageously used to heat the liquid 26 which, in turn,serves for supplying heat to the evaporator of the heat pump 76.

In similar manner, the other heat inputs may be advantageously connectedthrough heat storage tank 50 to many of the heat outlet units. Oneexample of each will be mentioned.

Air coil 56 can be used to melt ice in tank 50 created by operation ofheat pump 76.

Heat pump 58 can be used during night off-peak hours to melt at 118° F.a PCM in the tank 50, which will, in turn, provide heat during the dayto air coil in duct 74 or to radiant floor heater 80.

Electric heater 60 can supply heat to melt a PCM at 150° F. in tank 50during off-peak hours to be used during the day in baseboard radiator78.

Fossil fuel boiler 62, undersized for direct heating application in achurch, can store heat ahead of time in tank 50 and release it into thechurch on Sunday morning through radiant heater 80.

Ice rink 64 can be kept frozen during peak daytime hours by coolnessstored, for example in a PCM melting at 12° F., such as 22% ethyleneglycol and water, which was frozen the previous night by operation ofheat pump 76.

Cold storage room 66 may similarly be kept cold by storing coolness fromthe heat pump 76 during off-peak hours.

Cooling tower 86 can be operated at night to freeze at 55° F. a PCM andthereby supply air conditioning through air coil 56 in the daytime.

There are many other combinations for which thermal storage may be used.It will be understood that there may be multiple heat inlets andmultiple outlets which may be interconnected in various crosscombinations. FIG. 5 illustratively shows only some of the possible heatinput and heat output equipment that might be advantageously used.

FIG. 6 is an enlarged cross-sectional view of a section of the thermalstorage device 2. The section shows two layers of mat 8 spaced apartapproximately 1" by rubberized hair 22 which has an open wiryappearance. Rubberized hair has very large air spaces within the mattedstructure and actually leaves most of the space for PCM 24 while alsokeeping the layers of mat 8 spaced properly. The flexible tubing mat 8is factory fabricated using small diameter twin tubings 10 which areextruded out of thermoplastic material suitable for a wide range oftemperature and is corrosion resistant.

Areas 24A of PCM denote the frozen crystals around the tubes during adischarging cycle when the PCM is giving up heat. Areas 24B between thetubes show the melted unfrozen part. During a charging cycle this wouldbe reversed. Heat domain divider line 42 denotes the locationequidistant between the tubes where the heat flow divides between thedomain of each tube. It should also be noted that whichever of the tubes10 of each pair is connected to inlet header 27 will have more frozenPCM surrounding it during a discharge cycle because it is colder andwill have more melted PCM surrounding it during a charging, heatingcycle. FIG. 6 is shown near the halfway point (close to the U-bends) solittle difference in temperature is noted, and thus the frozen PCM 24Awill be fairly symmetrical.

It is to be understood that the charging or freezing period involvessolid PCM being around the tubes and melted PCM being out halfwaybetween the tubes, while the melting period involves melted PCM aroundthe tubes and frozen PCM at the halfway point. Since liquid can transferheat by conduction and convection; that is, moving around within itsmelted space, while solid PCM can only transfer by conduction, thefreezing-up period will take longer than the melting period except inthose cases where the solid salt may have a greater conductivity thanthe combined effect of the liquid conductivity and convection.

Prior researchers have found that stratification, as described under theforegoing list of problems, may also be greatly reduced by agitation orstirring. (See U.S. Pat. No. 2,677,664, dated May 4, 1954 to MariaTelkes.) This agitation or stirring is not feasible when salts areencapsulated in multiple containers nor in tanks not designed for it.

I have found that in my cylindrical tank, heat storage unit 2, with adepth of about 4 or 5 feet and a diameter from 2 to about 6 feet, thatpumping the liquid salt, when it is above its fusion temperature, downthrough a central vertical conduit tube located in the center of theheat exchanger spiral roll provides a strong recirculation that keepsmolten mixture nearly uniform throughout for preventing stratification.

FIG. 7 shows a vertical shaft column pump 88 having its motor mounted oncover 6 at the center of the cover with an impeller 90 designed to pumpliquid salt 24B vertically downward through conduit tube 92. The centralconduit 92 may be used as a rolling core about which the roll of thetubing mat 8, together with its spacer means, is rolled up. The lowerend of this conduit 92 is spaced from the bottom of the tank 4 by asuitable spacing, for example by a distance in the range fromapproximately one-half of the inside diameter of conduit 92 to one and aquarter of said inside diameter. The top end of the conduit tube 92 isspaced below the level of the liquid heat storage salt 24B by a distancecomparable to the spacing of its lower end above the bottom of the tank.The motor for the pump 88 is shown as a small electric motor having itsshaft directly connected to the propeller impeller 90. Above theimpeller is a shaft bearing support 91, such as a spider, attached tothe upper end of the conduit 92 and having multiple passageways as shownfor the liquid to pass down through the bearing support 91. The liquid(melted) salt 24B flows downwardly through the conduit 92 pumped by theaction of the propeller impeller 90, and at the bottom of tube 92 theflow spreads outward over the bottom of tank 4 and rises uniformly byrecirculation to the top of the conduit tube 92 where it reenters thetop of this conduit and is again pumped downwardly by the impeller 90.

The recirculating flow insures that the percentage mixture of water andsalt will be and will remain uniform throughout the tank during theliquid (melted) condition of the PCM, so that subsequent crystallizationwill be the same throughout.

As an example of the stratification problem which is thereby avoided, itis noted that in sodium thiosulphate pentahydrate (melting at 118° F.when pure or at 115° F. in a suitable commercial form containing someimpurities) there is a tendency when melted for an excess of water torise to the top of its container. The resulting scarcity of water at thebottom tends to cause formation of a crystal with less water, namely,the higher melting point dihydrate form, rather than the desiredpentahydrate crystal. Moreover, this tendency toward stratification withan excess of water rising to the top of the container is a progressivephenomenon. Thus, upon each successive melting and re-freezing cycle,more and more of the undesired dihydrate crystal tends to collect at thebottom of the container and more and more water tends to collect at thetop, until dihydrate crystal precipitate occupies about one-third of theentire volume of the material. Similar stratification problems areencountered with other compositions of PCM, as indicated above and asdescribed in the introductory portion of this specification.

Such stratification will not happen when the liquid (melted) PCM iscirculated within the tank 4 by pumping it in the manner as shown. Anelectric switch may be used for automatically turning the electric motorof the pump 88 on and off at the appropriate times.

In view of the fact that it is desirable to assure that the circulationpump 88 be extremely reliable in operation over long periods of time,there is an advantageous pump arrangement which may be used so as toobtain a very high degree of reliability. The electric motor is arelatively low power single-phase alternating current motor of theshaded-pole induction type. Such a shaded-pole motor has a relativelysmall starting torque and a relatively considerably increased operatingtorque when it is running at normal operating speeds.

Consequently, there is no need to use any on-off switch. The shaded-polemotor is continuously energized with alternating current, therebyavoiding any possible problems with switching it on and off. When thePCM becomes solidified around the impeller 90, the motor stalls andremains stalled until the PCM becomes melted around the impeller. Byvirtue of the low starting torque of the shaded-pole motor, it does notstrain against the solid-bound impeller and does not become over heated.Very little electric power is wasted. As soon as the localized volume ofthe PCM immediately surrounding the impeller has become melted, theimpeller automatically begins to rotate again as driven by thecontinuously energized motor, regardless of how long it may have beenbound by the solid (frozen) condition of the PCM.

FIG. 8 shows the grid-like flexible tubing mat 8 being rolled up with analternate less expensive form of spacing means than the matting 22,shown in FIG. 2. Plastic spacing members 94, shown as tubes ofrelatively large diameter or plastic pipes are secured to the twintubings 10 of the flexible mat 8 by adhesive or heat sealing ormechanical fastening. The spacing members 94 extend transversely to thelength of the tubing mat 8 and are spaced at intervals of approximately1 to 3 feet. They constitute a simple spacing means which is easilyfabricated and assembled.

FIG. 9 shows the rolled-up tubing grid with the tubing spacers 94 (FIG.8) installed in tank 4. Insulation 32, headers 16 and 18, tubing nipples17, and U-bends 20 are the same as shown in FIG. 3. Spacer tubes 94 maythus serve to replace the support strips 12 and 14 as shown in FIGS. 2and 3. Consequently, the tubing mat 8 comprises the twin tubings 10supported by the spacer members 94.

The twin tubings 10 each may have an inside diameter (ID) in the rangebetween approximately one-eighth of an inch and approximatelythree-eighths of an inch. It is to be understood that the tubingpassages may be oval shaped, if desired. In the illustrative examplesdiscussed above, the I.D. of each of the pair of tubes 10 isapproximately one-quarter of an inch.

EXAMPLE

I. An electric utility company introduces an off-peak rate of $0.060 perKWH for 10 A.M. to 10 P.M. and $0.015 from 10 P.M. to 10 A.M. A smalloffice building uses 20 tons of chilled water air conditioning at 44° F.from 9 A.M. to 6 P.M. during the summer season. The following will showthe storage equipment embodying the present invention which is requiredand how much operating cost is saved.

Ice at 32° F. provides 44° F. water with a 12° differential. The amountof ice required is computed as follows:

(a) 12 hours×20 tons×12,000 BTU's per hour per ton÷144 BTU's per poundof ice÷56 pounds ice per cubic foot requires 357 cubic feet of ice inthe PCM tanks 50 or 50A. A 6 ft. diameter plastic tank five feet high isa practical maximum size and this holds 120 cubic feet up to a 41/4 ft.level. Therefore, three 6 ft. diameter tanks are required.

The spacing of the plastic tubes within the ice must be such that allthe ice be melted in 9 hours and all the water refrozen in 12 hours,because there is a time period described above when the rates are low.The spacing determines the total heat transfer area and thus the lengthof the spiral tubing mat to be installed in the tank.

I have found that 14 BTU/hr/sq.ft./°F. can be transferred from an iceslab on both sides of the mat up to 1" thick.

Assuming an average temperature differential of 4° F. when freezing theice, it would mean the chilled antifreeze solution would enter at about24° F. and leave at 32° F. with an average of 28° F. A refrigerantsuction temperature of about 18° F. produces this desirable temperaturepattern.

The calculation, then, is as follows: ##EQU1## and with mats 4 ft. wide,this means 4286/4=1072 running ft. of mat located in three 6 ft.diameter tanks. Such tanks have a combined cross-sectional area of 85sq. ft. The spirally coiled mats take up substantially the whole spacein the cylindrical tanks. Thus, each coil path has a width of85/1072=0.079 ft. or 0.95 in. Therefore, 1072 running feet in the threetanks provide 357 running feet per tank.

I have found out that pressure drop considerations as a practical matterlimit mats to about 90 ft. in length. Thus, you install more than onemat in each tank, 4 mats per tank each 89 ft. long, or 12 89 ft. mats inall. The spacer material would be 0.95" less the width of the mat whichis 0.31", thus about 5/8". An alternative is to use 6 mats 60 ft. longwhich would raise the cost because of more headers and U-bends but wouldreduce pressure drop, allow for more flow, and provide faster response.

The present operating cost of the office building air conditioningequipment would be about: ##EQU2##

Assuming that this small office building is operating 25 weeks, 5 daysper week at 50% of full load 9 hours per day, the calculation is asfollows:

    25×5×9×0.5×20=11,250 ton-hrs., or $3,600 cost. (d)

Since the refrigerant suction temperature will be lower because thefreezing of the ice, about 18° F. vs. 34° F. for chilled water, theC.O.P. (coefficient of performance) of the chiller heat pump can beassumed to be about 2.5 instead of 3.0. The cost is calculated asfollows: ##EQU3## and for 11,250 ton-hrs., the cost would be $1,080. Asavings of $2,520 per year, or a savings of 70%, compared to operatingwithout storage in the small office building.

II. Example II using sodium thiosulfate pentahydrate, a phase changematerial, PCM-118, which melts at 118° F., or 115° F. in this casebecause of certain impurities, is to store heat for one cloudy day andtwo nights during 30° F. average temperature weather in a house thattakes 50,000 BTU's/hr at 0° F. Since the design base is 65° F.,35/65×50,000, or 26,923 BTU's per hour for 40 hours, or 1,076,920 BTU'smust be able to be stored.

With 92 BTU's per pound latent heat and 18 BTU's per pound sensible heatbetween 100° F. and 135° F., 1,076,920 BTU's divided by 110 BTU's perpound shows that 9,790 pounds of salt are required.

Since salts are more conveniently loaded at the factory into tank ortanks 50, the weight of loaded containers is a factor, and a practicallimit of about 1,000 pounds for shipping and moving into a housebasement is assumed. PCM-118, with a specific gravity of 1.6, will storeover 100,000 BTU's and weight about 1,000 pounds in a plastic container2 feet in diameter and 4 feet high.

Ten such tanks 50 are needed for this example, providing about 1,100,000BTU's. Heat transfer liquid from the solar collectors at about 130° F.enters the tank tubing leaving at 116° F. for an average of 123° F.,=8°F. above the fusion point of 115° F. Opposite from the other example,charging the tank 50 involves melting liquid around the tubes firstwhich speeds heat transfer by convectional motion of the liquid.

Assuming two sunny days to charge the tanks with heat, assuming 900BTU's per square foot per day from the collectors, and assuming 25,000BTU's per hour are needed to heat the house during the 8 hour sunnydays, then 1,276,000 BTU's would be needed along with1,476,000/900×2=820 sq. ft. of collector. The charging time is 16 hours(the two 8 hour solar days) and the discharging (freezing) time is 40hours.

Since PCM-115 has a thermal conductivity about 1/3 less than ice, anoverall coefficient of about 5 BTU/hr/ft² /°F. from each side isreasonable, or 10 overall when the salt is solid. The partially liquidphase should be higher but can be assumed to be the same. Thecalculation for the mat area would be: ##EQU4## and with 4 feet highmats, it would mean 210 running feet divided into 10 tanks, or 21 feetlength per mat. Since the ten tanks 50 are each 2.0' diameter, theirarea is 3.14 sq. ft., and 3.14 sq. ft./21 ft.=0.150 ft. or 1.8 in. isthe width of each coil path of the spiral. Subtracting the mat thicknessof 0.3 in., the spacer material is 1.5" thick.

It is to be noted that since the salt shrinks as it freezes, the moltensalt should more than cover the tubes and the frozen salt will betotally within the height of the tubes in tank 50.

The headers 16 are made of ABS (acrylonitrile butadiene styrene) or CPVC(chlorinated polyvinyl chloride) plastic pipe with ABS or CPVC nipples17 solvent cemented to the headers for low cost, adequate heatresistance and elimination of corrosion. The mat tubing is a medium orhigh density polyethylene with butyl rubber additive for flexibility toaid in making tight sealing joints. Stainless steel U-bends andstainless steel tubing clamps are the only metal in contact with thesale to avoid galvanic action.

In the above example of the house, it was mentioned that the PCM heatstorage tanks could be located in the basement. It is to be understoodthat such thermally insulated tanks can be mounted outside of a house orother building on a suitable foundation, such as a concrete pad.

EXAMPLES OF PCM's

Listed below are the various PCM materials which I have found to beuseful and economically feasible, together with their respecfiveapproximate melting points. As indicated above, the practicalcommercially available materials may contain minor amounts of impuritieswhich may slightly change the melting points, and so the followinglisting is to be understood from a practical point of view:

    ______________________________________                                        PCM               Approximate Melting Temp.                                   ______________________________________                                        Water and Ethylene                                                                              This PCM will cover the                                     Glycol Mixture    temperature range from 32° F.                                          down to -15° F., or lower,                                             depending upon the relative                                                   proportion of ethylene glycol                                                 in the mixture.                                             Potassium Chloride                                                                              12° F.                                               80% water by wt.                                                              20% salt by wt.                                                               Sodium Sulphate Decahydrate,                                                                    Sodium Sulphate Decahydrate                                  or mixed with one or more                                                                      melts at about 89° F. As                              of: Potassium Nitrate                                                                          explained in M. Telkes Pat.                                    Sodium Chloride                                                                              No. 3,986,969, the range                                       Ammonium Chloride                                                                            from 40° F. to 89° F. can be                     Potassium Chloride                                                                           covered by mixing it with one                                                 or more of the salts listed                                                   in the appropriate pro-                                                       portions                                                    Disodium Phosphate                                                                              97° F.                                               Dodecahydrate                                                                 Sodium Thiosulphate                                                                             118° F.                                              Pentahydrate                                                                  Trisodium Phosphate                                                                             150° F.                                              Dodecahydrate                                                                 Magnesium Chloride                                                                              243° F.                                              Hexahydrate                                                                   ______________________________________                                    

EXAMPLES OF HEAT TRANSFER LIQUIDS

For use below 32° F., a mixture of water and an antifreeze such asethylene glycol.

For use from 32° F. to about 210° F., water.

For use above the boiling point of water, there are a number ofcommercially available heat transfer liquids. Ethylene glycol can beused up to about 320° F. There is Dowtherm heat transfer liquidavailable from Dow; Therminol heat transfer liquid available fromMonsanto; and Caloria heat transfer liquid available from Exxon, and soforth.

Another Example of Cold Storage

Another example of an application where off-peak cold storage can beused to advantage is for chilling the freezer cases in a supermarket.The PCM material which can be used effectively for this application ispotassium chloride hexahydrate which melts at 12° F. A suitable quantityof this material is frozen overnight when reduced costs for electricalpower are available. Then, during the daylight hours, when electricalrates are higher, a mixture of water and ethylene glycol is used as theheat transfer liquid to chill the various freezer cases.

The examples given are illustrative of various applications that may bemade of phase change material thermal energy storage according to myinvention. These examples are not to be thought of as limiting as to anyparticular use, dimension, or material. It is intended that variousmodifications which might readily suggest themselves to those skilled inthe art be covered by the scope of the following claims.

I claim:
 1. Heat storage tank apparatus for temporarily storing heatenergy comprising:a thermally insulated tank providing convenient accessto its interior, liquid supply and return connections extending withinsaid tank, a liquid-conducting heat exchanger in said tank connected tosaid connections, a melting/freezing phase change material comprising ahydratable salt and water substantially filling said tank in contactwith said heat exchanger, said heat exchanger being distributed throughsubstantially the entire volume of said tank for freezing saidhydratable salt and water into its solid phase throughout substantiallythe entire volume of said tank, said heat insulated tank and said heatexchanger being formed of materials capable of withstanding temperatureabove and below the melting/freezing phase change temperature of saidhydratable salt and water, said material being subject to stratificationwhen in its melted condition, means for preventing stratification of thephase change material when in its liquid state including: impellingmeans for circulating the melted material in the tank, motor meansconnected to said impelling means for driving the impelling means whenthe phase change material is melted for recirculating the meltedmaterial for insuring that the salt and water mixture will remainuniform throughout the melted mixture so that crystallization will bethe same throughout the tank during subsequent freezing, said motormeans being a relatively low power electric motor having a relativelylow starting torque, and electric power being fed to said motor forautomatically causing the impeller means to operate when the materialsurrounding the impelling means is melted.
 2. Heat storage tankapparatus as claimed in claim 1, in which:said impelling means has aconduit associated therewith extending in an upright direction in thetank, said conduit communicates at its lower end near the bottom of thetank with the interior of the tank, and said impelling means is arrangedto impel melted phase change material downwardly through said conduitfor causing the melted material to exit from the lower end of saidconduit and then to rise through the tank so as to return to theimpelling means to be recirculated.
 3. Heat storage tank apparatus asclaimed in claim 1, in which:said tank is generally cylindrical inconfiguration and has its axis upright, said impelling means has arecirculation conduit associated therewith extending upright along theaxis of the tank, said conduit communicates at its upper and lower endswith the phase change material in the top and bottom of the tank,respectively, said impelling means is arranged to impel melted phasechange material downwardly through said conduit for causing the meltedmaterial to rise through the volume of the tank surrounding said conduitfor producing recirculation thereof back to the upper end of saidconduit, said heat exchanger in said tank includes closely spaced smalldiameter flexible conduits arranged in a roll surrounding saidrecirculation conduit with many spiral turns, and spacing meansapproximately uniformly horizontally spacing the respective turns ofsaid roll.
 4. Heat storage tank apparatus for temporarily storing heatenergy comprising:a thermally insulated tank, a liquid-conducting heatexchanger in said tank having supply and return connections thereto forfeeding liquid into and out of said heat exchanger, a liquid/solid phasechange material substantially filling said tank in contact with saidheat exchanger, said insulated tank and said heat exchanger being formedof materials capable of withstanding temperature above and below thephase change temperature of said liquid/solid phase change material,said liquid/solid phase change material being subject to stratificationwhen in its liquid phase, impelling means for circulating the liquid inthe tank for preventing stratification, motor means connected to saidimpelling means for driving the impelling means when the phase changematerial is in its liquid state, and said motor means is a relativelylow power electric motor having a relatively low starting torque so thatsaid motor and said impelling means are not damaged when the motor isstalled by the freezing of the phase change material around saidimpelling means.
 5. Heat storage tank apparatus as claimed in claim 4,in which:said impelling means has a conduit associated therewithextending in an upright direction in the tank, said conduit communicatesat its lower end near the bottom of the tank with the interior of thetank, and said impelling means is arranged to impel liquid phase changematerial downwardly through said conduit for causing the liquid phasechange material to exit from the lower end of said conduit and then torise through the tank so as to return to the impelling means to berecirculated.
 6. Heat storage tank apparatus as claimed in claim 4, inwhich:said tank is generally cylindrical in configuration and has itsaxis upright, said impelling means has a recirculation conduitassociated therewith extending upright along the axis of the tank, saidconduit communicates at its upper and lower ends with the phase changematerial in the top and bottom of the tank, respectively, said impellingmeans is arranged to impel liquid phase change material downwardlythrough said conduit for causing the liquid phase change material torise through the volume of the tank surrounding said conduit forproducing recirculation thereof back to the upper end of said conduit,and said heat exchanger is arranged in a spiral design in said tankencircling said upright recirculation conduit.
 7. Heat storage tankapparatus for temporarily storing heat energy comprising:a thermallyinsulated tank having a fill port in the top thereof, liquid supply andreturn headers extending within said tank, at least one grid ofclosely-spaced small diameter conduits of flexible plastic materialconnected at their ends to the respective headers and arranged withinsaid tank to give multiple parallel liquid circuits through which aheat-transfer liquid can flow, a liquid/solid phase change materialsubstantially filling said tank and surrounding said grid ofclosely-spaced small diameter conduits, spacer means in said tankassociated with said grid for providing substantially uniform horizontalspacing of said small diamter conduits of plastic material andpermitting free flow of the phase change material in its liquid statethroughout said tank, said insulated tank, said flexible plasticconduits and said headers being formed of materials capable ofwithstanding temperature above and below the melting point of saidliquid/solid phase change material, said grid being rolled into a spiralroll configuration as seen in cross section looking downwardly, and saidspacer means is low density matting having multiple spaces therein whichhas been rolled into said spiral roll configuration together with saidgrid, said matting and said grid being in alternate spiral layers asseen in cross section, whereby said phase change material may bealternately melted and frozen throughout the interior of the massthereof as heat is added thereto or withdrawn therefrom by circulatingheat-transfer liquid through said conduit grid.
 8. Heat storage tankapparatus for temporarily storing heat energy comprising:a thermallyinsulated tank having a fill port in the top thereof, liquid supply andreturn headers extending within said tank, at least one grid ofclosely-spaced small diameter conduits of flexible plastic materialconnected at their ends to the respective headers and arranged withinsaid tank to give multiple parallel liquid circuits through which aheat-transfer liquid can flow, a liquid/solid phase change materialsubstantially filling said tank and surrounding said grid ofclosely-spaced small diameter conduits, spacer means in said tankassociated with said grid for providing substantially uniform horizontalspacing of said small diameter conduits of plastic material andpermitting free flow of the phase change material in its liquid statethroughout said tank, said insulated tank, said flexible plasticconduits and said headers being formed of materials capable ofwithstanding temperature above and below the melting point of saidliquid/solid phase change material, said grid being rolled into a spiralroll configuration as seen in cross section looking downwardly, and saidliquid supply and return headers extending generally vertically withinsaid tank, and each of said conduits extends generally in a horizontalplane in a spiral having a reversal of direction near the center of saidspiral roll configuration with the liquid flow in each conduittravelling generally in a horizontal plane from the supply header alongan inward spiral path through the phase change material to said reversalof direction and then travelling in a generally horizontal plane alongan outward spiral path through the phase change material, said outwardspiral path being near to said inward spiral path.
 9. Heat storage tankapparatus for temporarily storing heat energy comprising:a thermallyinsulated tank, a liquid-conducting heat exchanger in said tank havingsupply and return connections thereto for feeding liquid into and out ofsaid heat exchanger, a liquid/solid phase change material substantiallyfilling said tank in contact with said heat exchanger, said heatexchanger substantially filling the tank for causing each region in thetank to be no more than a short distance away from the heat exchangerfor freezing solid said phase change material throughout substantiallythe whole tank, said insulated tank and said heat exchanger being formedof materials capable of withstanding temperature above and below thephase change temperature of said liquid/solid phase change material,said liquid/solid phase change material being subject to stratificationwhen in its liquid phase, impelling means for circulating the liquid inthe tank for preventing stratification, motor means connected to saidimpelling means for driving the impelling means when the phase changematerial is in its liquid state, an upright central conduit in said tankassociated with said impelling means and extending through said heatexchanger, said conduit communicating with said phase change materialnear the bottom of said tank and communicating with said phase changematerial near the top of said tank, and said impelling means impellingthe phase change material through said upright conduit when saidmaterial is in its liquid state for said liquid to be recirculatedthrough tank for preventing stratification of said material when in itsliquid state.
 10. Heat storage tank apparatus as claimed in claim 9, inwhich:said liquid/solid phase change material is a hydratable salt, andsaid impelling of the liquid phase change material through said uprightconduit insures that the percentage mixture of salt and water remainssubstantially uniform throughout the tank during the liquid statecondition of said hydratable salt.